Field of the Invention
The present invention is directed to efficient and low cost recovery of natural gas liquids (NGL) from hydrocarbon gas streams.
Description of the Prior Art
Cryogenic processes are commonly used to recover heavy hydrocarbons from feed gases. By cooling a natural gas, it is possible to condense a portion of the heavy hydrocarbons, which can then be separated. The amount of NGL that is formed is a function of the gas composition, pressure, and temperature to which the gas is cooled. Greater quantities of NGL can be recovered by cooling to colder temperatures, but greater initial investment and ongoing operating expense is required to provide the required refrigeration and pre-treatment.
Refrigeration for the cooling can be supplied by expansion of the gas to lower pressure by the Joule-Thomson effect. Often, the processed gas must then be re-compressed to be delivered into the sales gas pipeline system. This re-compression can represent a significant capital and operating expense.
Depending on natural gas and NGL product pricing, it may not be economical to use highly cryogenic technologies to extract the hydrocarbon liquids. However, in many cases some NGL extraction is required so that the natural gas will meet pipeline specifications such as maximum heating value limitations. In this scenario, a less capital intensive technology is needed.
Simple low temperature separation (LTS) processes have been used for many years in applications where modest NGL recovery is required (see FIG. 1). In such processes, mechanical refrigeration systems are commonly used to provide the required cooling. A significant advantage of using mechanical refrigeration instead of expansion to provide cooling is that the majority of the product gas is typically returned at only slightly lower pressure than the arriving feed gas. This reduces or eliminates re-compression of the gas that is usually required for an expansion-based technology. Further, the more moderate operating temperatures may eliminate the need for certain pre-treatment processes (e.g., carbon dioxide removal), allow for the use of less expensive pre-treatment systems (e.g., glycol dehydration vs. molecular sieve dehydration), and require less usage of alloy metallurgy in piping and equipment, which may further reduce the required investment.
One drawback of traditional LTS processes is a lack of selectivity in the NGL components recovered. Propane, butanes, pentanes, and heavier hydrocarbons each have different product pricing with the heavier components typically having a higher value than the lighter components in the NGL. Therefore, it is economically desirable to recover the heaviest components in the NGL until the sales gas pipeline specifications are met. However, conventional LTS processes do not have means to control the recovery of specific components, and product recovery is largely a function of gas composition at a particular temperature and pressure.
Another drawback of traditional LTS processes is re-compression of the vapor generated during stabilization of the NGL product. The high pressure NGL stream typically contains a significant quantity of lighter components (such as methane) that are also condensed during the chilling process. Heat is supplied in a fractionation tower to remove these light components from the product liquid to meet NGL specifications. These light components are commonly used to satisfy fuel requirements for the facility, but excess gas must be re-compressed and blended into the feed or sales gas stream.
What is needed is a means to improve recovery of the more valuable heavy components from the feed gas and to reduce the quantity of light components that must be re-compressed.